Corteva Agriscience, the newly formed agriculture division of DowDuPont, will license two insect traits from Monsanto, an aboveground Bt protein stack and Corn Rootworm III, which contains an RNAi rootworm trait.

The RNAi rootworm trait will prove especially valuable to Corteva, as it is the only novel mode of action against western corn rootworm, which has shown varying levels of resistance to every Bt protein on the market targeting it.

Corteva will stack both of the licensed Monsanto insect traits with its own Bt proteins and Enlist technology, which allows corn plants to tolerate 2,4-D choline, FOP herbicides and glyphosate. That means by the early 2020s, farmers in the U.S. and Canada should have access to the RNAi rootworm trait from two major seed companies, Corteva and Bayer-Monsanto.

This is good news for corngrowers, said Texas A&M Extension entomologist Pat Porter.

“This will help delay resistance to the traditional Cry toxins that are still working, and the Cry toxins that are still working will help delay resistance to the RNAi,” he told DTN.

THE RNAI ROOTWORM TRAIT

Bt corn works by expressing Bt proteins that, when ingested by the rootworm, target receptors in its gut and kill it.

Monsanto’s RNAi trait works differently. When ingested by a corn rootworm, it switches off a gene in the rootworm’s DNA, which halts the production of a key protein, killing the insect.

“The nice thing about it is that, unlike Bt proteins, it’s able to control even larger [rootworm] larvae as well small ones,” said Tom Clark, Monsanto’s global corn insect platform lead. “It’s a very different mode of action than any Bt technology out there today.”

However, like Bt, the RNAi trait could face insect resistance in the field someday. In fact, Monsanto researchers have already generated rootworm resistance to the trait, in order to study it.

“Bugs always seem to win, so that’s a constant challenge for us as an industry,” said Tony Klemm, global corn portfolio leader for Corteva. He said Corteva will use 5% refuge-in-a-bag in its future use of the RNAi trait, as well as stacking it with multiple Bt proteins, to slow the development of resistance.

THE STATE OF INSECT RESISTANCE

Monsanto’s Corn Rootworm III event expresses both the RNAi trait and Cry3Bb1, the oldest rootworm Bt protein on the market. The other event Monsanto licensed to Corteva is a molecular stack of aboveground Bt proteins, Cry1A.105 + Cry2Ab2, already in use in Bt corn and Bt cotton varieties.

With these traits in hand, pending regulatory approvals, both Corteva and Monsanto will release SmartStax PRO corn hybrids in the early 2020s that express all of the following traits.

Corteva will add the Enlist trait to these and market that as a separate product, unnamed for now.

That may look like a lot of insect protection — and it is. However, scientists have documented insect resistance in either corn or cotton to all the Bt proteins listed.

See DTN reporting on western corn rootworm resistance to the Bt rootworm proteins here.

See DTN reporting on western bean cutworm resistance to Cry1F, the Herculex trait, here.

And finally, see DTN reporting on problems with Cry1A.105 x Cry2Ab2 here.

That means growers will need to be careful stewards of the current Bt protein offerings now and after the RNAi-stacked hybrids come to market, Clark said.

“One thing we do at Monsanto is recommend using best management practices, where we encourage growers to use crop rotation, rotating out of Bt traits, and not utilizing the same Bt traits over and over,” he said. “We’re going to continue to keep that message going forward. But if best management practices are not adhered to, of course there will be the risk for more [insect resistance].”